1. Field of the Invention
This invention relates generally to members having a high stiffness such as used in electro-acoustical transducers. In particular, this invention relates to an improved lightweight, high stiffness speaker diaphram member and a method for fabricating such a member.
2. Description of the Prior Art
Loudspeaker diaphrams are well-known in the prior art and are used to match the relatively low acoustic impedance of air to the relatively high mechanical impedance of a loudspeaker drive system. The driver for a loudspeaker generally can deliver a force of large magnitude but delivers a relatively small displacement. A loudspeaker diaphram having a large area provides the means by which a large volume of air can be moved by a driver having a relatively small displacment.
It is desirable to use material as strong and as light as possible for a speaker diaphram so that minimal mechanical energy is used to accelerate the diaphram mass, providing good transient response and preventing buckling of the diaphram under heavy loading.
The specific stiffness modulus S of a material is a measure of resistance to deflection of a unit weight of material. The specific stiffness modulus of a material is defined as the ratio of the modulus of elasticity Q, Young's modulus, to the density .rho. of the material. EQU S = .rho./Q (1)
loudspeakers using presently available diaphram materials do not lend themselves to miniaturization by reduction of the cone diameter. This is because reduction of the cone diameter results in loss of efficiency and poorer low frequency response. It is desirable and theoretically possible to reduce the height of a speaker cone in order to reduce the overall volume of a loudspeaker without losing efficiency and low frequency response. In order to most effectively reduce cone height and still obtain the same efficiency and low frequency response as current loudspeakers, it is necessary to use speaker cone materials having improved properties such as contemplated by the instant invention.
Speaker diaphrams may be made from solid metals, such as beryllium, which has a very high specific stiffness S but which is also very expensive and toxic. Other metals have relatively high densities .rho. and, consequently, have relatively low specific stiffness moduluses S.
Paper is commonly used as a speaker diaphram material. The cellulose fibers of paper are inexpensive, have a relatively high specific stiffness modulus S, have a high tensile strength, bond to each other without binders, may be formed to certain configuration, and may be formed to be relatively gastight. One of the disadvantages of paper as a cone material is its sensitivity to humidity and to heat, causing cones made of such material to eventually deteriorate. Impregnation or coating with a plastic humidity-resistant material adds undesirable weight.
Paper speaker cones are presently made by two processes, a felting process and and seamed construction process. The felting process involves applying a controlled vacuum to a perforated suction mold of the desired cone shape which is immersed in a solution of cellulose fibers. A layer of randomly oriented fibers is deposited on the mold. The felted shape is then pressed, dried, and trimmed. The seamed construction process involves forming a cone with a sheet of paper and gluing the overlapping edges together. The performance of a cone having a seam is somewhat inferior to that of a seamless type and the shape of such a speaker is limited to either straight or large radius cross sections.